U.S. patent number 5,508,576 [Application Number 08/343,860] was granted by the patent office on 1996-04-16 for rotor for brushless electromotor.
This patent grant is currently assigned to Seiko Epson Corporation. Invention is credited to Kenichi Endo, Yoshikazu Koike, Takashi Nagate, Takeshi Seto, Yoshihiko Yamagishi.
United States Patent |
5,508,576 |
Nagate , et al. |
April 16, 1996 |
Rotor for brushless electromotor
Abstract
A rotor having permanent magnets for a brushless electromotor
includes a yoke formed from a plurality of silicon steel sheets
laminated together and provided along its outer periphery with even
number of, at least four magnetic poles. The magnetic poles are
alternately provided substantially at equal distances from a
rotational axis with slots to receive the respective permanent
magnets such that the sides of the permanent magnets facing a
rotary shaft have the same polarity. As a result, a compact and
efficient rotor is obtained, and damage to or flying off of the
permanent magnets is prevented.
Inventors: |
Nagate; Takashi (Suwa,
JP), Endo; Kenichi (Suwa, JP), Koike;
Yoshikazu (Suwa, JP), Seto; Takeshi (Suwa,
JP), Yamagishi; Yoshihiko (Suwa, JP) |
Assignee: |
Seiko Epson Corporation (Tokyo,
JP)
|
Family
ID: |
27581901 |
Appl.
No.: |
08/343,860 |
Filed: |
November 17, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
983585 |
Feb 4, 1993 |
5369325 |
|
|
|
Current U.S.
Class: |
310/156.54;
310/64 |
Current CPC
Class: |
H02K
15/03 (20130101); C08L 67/02 (20130101); H02K
1/2746 (20130101); H02K 11/0141 (20200801); C08L
23/02 (20130101); C08L 77/00 (20130101); H02K
9/20 (20130101); C08L 23/02 (20130101); C08L
2666/20 (20130101); C08L 67/02 (20130101); C08L
77/00 (20130101); C08L 67/02 (20130101); C08L
2666/02 (20130101); C08L 77/00 (20130101); C08L
2666/02 (20130101); C08L 77/00 (20130101); C08L
2666/18 (20130101); C08L 77/00 (20130101); C08L
2666/04 (20130101); C08L 23/16 (20130101); H02K
7/04 (20130101) |
Current International
Class: |
C08L
77/00 (20060101); C08L 67/00 (20060101); C08L
67/02 (20060101); H02K 1/27 (20060101); C08L
23/00 (20060101); C08L 23/02 (20060101); H02K
9/19 (20060101); H02K 15/03 (20060101); H02K
9/20 (20060101); H02K 11/00 (20060101); H02K
7/04 (20060101); C08L 23/16 (20060101); H02K
7/00 (20060101); H02K 021/12 (); H02K 001/32 () |
Field of
Search: |
;310/156,261,64,187
;29/598 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: LaBalle; Clayton E.
Attorney, Agent or Firm: Kanesaka & Takeuchi
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATION
This is a divisional application of Ser. No. 983,585 filed Feb. 4,
1993, now U.S. Pat. No. 5,369,325.
Claims
We claim:
1. A rotor having permanent magnets for a brushless electromotor,
comprising:
a yoke made from a plurality of silicon steel sheets laminated
together so as to have an even number of magnetic poles;
at least one slot provided in one of said magnetic poles;
at least one permanent magnet provided in said slot; and
at least one slit extending inwardly from a periphery of said one
of said magnetic poles along a direction of magnetic flux for
providing a uniform distribution of magnetic flux on said permanent
magnet, wherein said one of said magnetic poles is provided with a
pair of bridges on opposite sides of said slot for connecting a tip
portion to a base portion of said one of said magnetic poles, said
bridges are provided on an outside thereof with a groove for
limiting passage of magnetic flux.
2. A rotor having permanent magnets for a brushless electromotor,
comprising:
a yoke made from a plurality of silicon steel sheets laminated
together so as to have an even number of magnetic poles;
at least one slot provided in one of said magnetic poles;
at least one permanent magnet provided in said slot; and
at least one slit extending inwardly from a periphery of said one
of said magnetic poles along a direction of magnetic flux for
providing a uniform distribution of magnetic flux on said permanent
magnet, wherein said one of said magnetic poles is provided with a
single bridge on a forward side of said slot with respect to a
direction of rotation.
3. A rotor according to claim 2, wherein said single bridge has
such a width that a leakage flux flows therethrough and said
permanent magnet has such a thickness that a leakage flux is
generated on opposite sides thereof.
4. A rotor having permanent magnets for a brushless electromotor,
comprising:
a yoke made from a plurality of silicon steel sheets laminated
together so as to have an even number of magnetic poles;
a shaft provided through said yoke;
at least one slot provided in one of said magnetic poles;
at least one permanent magnet provided in said slot; and
at least one heat pipe provided in said yoke in thermal contact
with said permanent magnet.
5. A rotor according to claim 4, which further comprises a through
hole provided in said yoke along said shaft at a position closer to
said slot than said shaft.
Description
FIELD OF THE INVENTION
The present invention relates to brushless electromotors suitable
for operation at high speed with high efficiency and more
particularly to a rotor for use with such brushless
electromotors.
BACKGROUND OF THE INVENTION
The brushless electromotors generally have a cylindrical rotor
provided on the outer peripheral surface thereof with permanent
magnets made of ferrite or the like.
FIG. 28 shows a conventional brushless electromotor. The brushless
electromotor 1 includes a motor casing (i.e. stator) 2 which
consists of a cylindrical side wall 3, and a front face plate 4 and
a rear face plate 5, both of which are employed to close opposite
ends of the side wall 3. Inside the side wall 3, there are provided
a plurality of driving coils 6 arranged in a cylindrical form and
fixed to the inner surface of the side wall 3. A rotary shaft 8 is
fixed concentrically to a rotor 7. The rotary shaft 8 projects from
opposite ends of the rotor 7 so that it is supported at one end in
a bearing 10 held in an opening 9 of the rear face plate 5 and at
the other end in a bearing 12 held in an opening 11 of the front
face plate 4. An annular member 13 is provided inside the side wall
3 of the motor casing 2 to support a plurality of magnetic pole
sensors 14 closely adjacent to one end surface of the rotor 7.
In FIG. 29, the rotary shaft 8 is inserted into and integrated with
a cylindrical yoke 70. The cylindrical yoke 70 carries on its outer
peripheral surface a pair of arcuate permanent magnets 71
magnetized to have N-poles on their outer sides and S-poles on
their inner sides and another pair of arcuate permanent magnets 72
magnetized to have S-poles on their outer sides and N-poles on
their inner sides. The respective pairs of permanent magnets 71 and
72 are alternately arranged around the yoke 70 and bonded
thereto.
In this brushless electromotor 1, the magnetic pole sensors 14
detect positions of the magnetic poles of the rotor 7 and, in
response thereto, a control circuit (not shown) supplies the
corresponding driving coils 6 with electric current so that an
interaction of electric current and magnetic flux causes the rotor
7 to be rotated. As it has been rotated in this manner, the rotor 7
now presents new magnetic pole positions to be detected by the
magnetic pole sensors 14 again, and the control circuit supplies
the other driving coils 6 with electric current, causing the rotor
7 to be rotated again. Such operation is repeated and thereby the
rotor 7 is continuously rotated. The rotary force thus generated is
taken out as a motive power from the electric motor by way of the
rotatable shaft 8.
FIG. 30 shows another rotor 7, in which the permanent magnets 71
and 72 are covered with a protective member 73 of nonmagnetic metal
to prevent these permanent magnets 71 and 72 from flying off due to
centrifugal force as the electromotor 1 is rotated at high
speeds.
However, the maximum energy product and the residual flux density
in the brushless electromotor utilizing the ferrite magnets are 3.3
MGOe and 2.8 KG, respectively, which are so low that it is
necessary to increase the permeance of the magnetic circuit to
develop a sufficient torque to drive the electromotor. As a result,
it is necessary to use a large amount of magnets, making the
electromotor disadvantageously bulky.
When the electromotor is used in the scroll type compressor or the
like for high speed rotations, the permanent magnets can be
destroyed or flying off because the stress generated by the
centrifugal force becomes greater than the material strength of the
permanent magnets and the adhesion of the magnets to the rotor.
Furthermore, covering the rotor with the protective member to avoid
the flying-off of the permanent magnets not only complicates the
manufacturing process of the rotor but also increases the gap
between the rotor and the stator by the thickness of the protective
member, and correspondingly increases the magnetic resistance. In a
consequence, the magnet density decreases and the efficiency is
significantly lowered.
The present invention has been developed to solve such problems as
described above with respect to the conventional brushless
electromotor.
Accordingly, it is an object of the invention to provide a compact
rotor for use in the brushless electromotor, which can be
manufactured efficiently and rotate at high speeds without the
danger that the permanent magnets are destroyed or fly off during
high speed rotations.
DESCRIPTION OF THE INVENTION
According to the invention there is provided a rotor having
permanent magnets for use in a brushless electromotor, which
includes a yoke formed from a plurality of silicon steel sheets
laminated together and provided along its outer periphery with even
number of, at least four magnetic poles, the magnetic poles being
alternately provided at substantially equal distances from a rotary
axis with slots to accommodate the respective permanent magnets so
that the sides of the permanent magnets facing the rotary shaft
have the same polarity.
With such arrangement, mutual repulsion of the diametrically
opposed magnetic poles provides the rotor with magnetic poles twice
the number of the permanent magnets.
The permanent magnets are inserted into the respective slots and
held radially between the high-permeability materials so that
flying-off of the permanent magnets due to high speed rotations is
avoided. Thus, the need for the member covering the outer periphery
of the rotor to prevent the permanent magnets from flying off is
eliminated, minimizing the iron loss resulting from the use of such
a covering member. The iron loss is further reduced by forming the
yoke from the laminated steel sheets.
The permanent magnets according to the invention have a simple
configuration and require no high precision in finishing the
surface. This facilitates the manufacture of the permanent
magnets.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical section of a brushless electromotor using a
permanent magnet rotor according to the first embodiment of the
invention;
FIG. 2 is a perspective view of the permanent magnet rotor
thereof;
FIG. 3 is a front view of a silicon steel sheet of the permanent
magnet rotor;
FIG. 4 is a schematic view showing lines of magnetic force
generated in the permanent magnet rotor which is incorporated into
the brushless electromotor;
FIG. 5 is a front view of a permanent magnet rotor in which the
magnetic poles have different widths according to the second
embodiment of the invention;
FIG. 6 is a front view of a permanent magnet rotor in which each
magnetic pole is provided with a plurality of slits according to
the third embodiment of the invention;
FIG. 7 is a schematic view of lines of magnetic force generated in
the permanent magnet rotor with the slits which is incorporated in
the brushless electromotor;
FIG. 8 is a front view of a permanent magnet rotor having six
magnetic poles according to the fourth embodiment according to the
fourth embodiment of the invention;
FIG. 9 is a sectional view showing, in an enlarged scale, a bridge
formed in a permanent magnet rotor according to the fifth
embodiment of the invention;
FIG. 10 is a plan view of a silicon steel sheet according to the
sixth embodiment of the invention;
FIG. 11 is a magnetic field analysis diagram showing a pattern of
lines of magnetic flux under a load torque;
FIG. 12 is an exploded perspective view of a permanent magnet rotor
according to the seventh embodiment of the invention;
FIG. 13 is a sectional view of the rotor of FIG. 12 as
assembled;
FIG. 14 is a diagram showing lines of magnetic flux observed after
the components are assembled to define a magnetic circuit;
FIG. 15 is a sectional view, taken along the axis, of a brushless
electromotor utilizing the permanent magnet rotor according to the
first embodiment;
FIG. 16 is a perspective view of a permanent magnet rotor according
to the eighth embodiment of the invention;
FIG. 17 is a sectional view, taken along the axis, of a brushless
electromotor utilizing the permanent magnet rotor according to the
eighth embodiment;
FIG. 18 is an exploded perspective view of a permanent magnet rotor
according to the ninth embodiment of the invention;
FIG. 19 is a sectional view of the permanent magnet of FIG. 18 as
viewed transversely to the rotary shaft thereof;
FIG. 20 is a sectional view of part of a yoke for the permanent
magnet rotor in an enlarged scale transversely to the axis
thereof;
FIG. 21 is a sectional view of part of another yoke for the
permanent magnet rotor in an enlarged scale transversely to the
axis thereof;
FIG. 22 is a sectional view, taken along the axis, of a permanent
magnet rotor according to the tenth embodiment of the
invention;
FIG. 23 is a perspective view of a permanent magnet rotor according
to the eleventh embodiment of the invention;
FIG. 24 is a perspective view of a variant of this permanent magnet
rotor;
FIG. 25 is a transverse sectional view of the rotary shaft for the
this embodiment;
FIG. 26 is a sectional side view of this permanent magnet rotor,
illustrating one of steps in manufacturing;
FIG. 27 is a perspective view of a skewed permanent magnet
rotor;
FIG. 28 is a longitudinal section of a conventional brushless
electromotor;
FIG. 29 is a perspective view of a conventional permanent magnet
rotor; and
FIG. 30 is a perspective view of another conventional permanent
magnet rotor which is provided with a protective member.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Initially, the first embodiment will be described in reference with
FIGS. 1 through 4.
In FIG. 1, the brushless electromotor 20 includes a motor casing
(i.e. stator) 2 which consists of a cylindrical side wall 3, a
front face plate 4, and a rear face plate 5. A plurality of driving
coils 6 are arranged inside the side wall 3 to define a cylindrical
array and fixed to the inner surface of the side wall 3. A rotary
shaft 8 is fixed concentrically to the rotor 7. The rotary shaft 8
projects from opposite ends of the rotor 7 as to be rotatably
supported at one end in a bearing 10 carried by the rear face plate
5 and at the other end in a bearing 12 carried by the front face
plate 4 of the motor casing 2. An annular support member 13 is
provided inside the side wall 3 to hold a plurality of magnetic
pole sensors 14 such that these sensors 14 are positioned closely
adjacent to one end surface of the rotor 7.
In this brushless electromotor 20, the magnetic pole sensors 14
detect positions of the magnetic poles of the rotor 7 and, in
response thereto, a control circuit (not shown) supplies the
corresponding driving coils 6 with electric current so that the
interaction of electric current and magnetic flux causes the rotor
7 to be rotated. As it is rotated, the rotor 7 presents new
magnetic pole positions to be detected by the magnetic pole sensors
14 again, and the control circuit supplies the other driving coils
6 with electric current, causing the rotor 7 to be rotated again.
Such operation is repeated to continuously rotate the rotor 7. The
rotary force thus generated is taken out as a motive power from the
electromotor via the rotary shaft 8.
FIG. 2 shows the same rotor 7 as that incorporated in the brushless
electromotor 20 of FIG. 1, and FIG. 3 shows a silicon steel sheet
22 constituting the rotor 7. A yoke 21 of the rotor 7 consists of a
plurality of such silicon steel sheets 22 laminated one upon
another in the axial direction of the rotary shaft 8 and integrally
joined by pressed rectangular recesses 23 of the respective sheets
22 forcibly engaged with each other.
Each silicon steel sheet 22 made from a high permeability material
has its surface coated with an inorganic insulating film and has a
thickness of either 0.35 mm or 0.5 mm. As shown in FIG. 3, the
silicon steel sheet 22 has four magnetic poles 24a and 24b radially
extending to their arcuate outer ends and successively spaced from
one another by an angle of 90.degree.. A pair of diametrically
opposed magnetic poles 24a are provided symmetrically to the
rotational axis with corresponding pair of slots 25 to accommodate
respective permanent magnets 30 and 31. Since the pair of magnetic
poles 24a are provided with the pair of slots 25, respectively, the
outer end portion and the root portion of each magnetic pole 24a
are connected to each other by narrow bridges 26 defined on
laterally opposite ends of each slot 25. Each silicon steel sheet
22 is centrally provided with an opening 27 to receive the rotary
shaft 8, and this opening 27 is provided along part of its
periphery with a keyway 28.
The rotary shaft 8 is diameter-enlarged along its longitudinally
intermediate portion such that it snugly fits in the opening 27.
After the silicon steel sheets 22 are integrally laminated to form
the yoke 21, the rotary shaft 8 is inserted into the opening 27.
The enlarged intermediate portion of the rotary shaft 8 is provided
with a key 29 which engages the keyway 28 so that the rotor 7
cannot be rotated apart from the rotary shaft 8.
While the yoke 21 is composed of the silicon steel sheets 22, the
silicon steel sheets 22 may be replaced by cold rolled steel sheets
(SPCC) to form the yoke 21.
A pair of permanent magnets 30 and 31 are then inserted into the
pair of slots 25 with their N-poles being diametrically opposed to
each other. Consequently, the permanent magnets 30 and 31 are
flanked in the radial direction by the high-permeability silicon
steel sheets 22. Since the N-poles of these permanent magnets 30
and 31 are opposed to each other and repel each other, the magnetic
poles 24a are S-polarized while the magnetic poles 24b are
N-polarized, thus providing a 4-pole rotor.
FIG. 4 shows a pattern of magnetic force lines generated by the
rotor 7 incorporated into the electromotor. As shown, the lines of
magnetic force exiting from the N-pole of the permanent magnet 30
pass through the bridges 26 toward the S-pole. The bridges 26 are
made so narrow that the magnetic flux density therein is readily
saturated. A notch 24' is provided between the S-pole 24a and the
N-pole 24b so that the lines of magnetic force run from the
pole-face of the magnetic pole 24b through the inside of the
driving coils 6 to the pole-face of the magnetic pole 24b to the
S-pole under the mutual repulsion of the diametrically opposed same
poles of the permanent magnets 30 and 31. The pressed portions 23
are rectangular and their longer sides are angled by 45.degree.
with respect to a direction in which the magnetism of the rotor 7
is oriented so as not to interfere with the lines of magnetic
force.
According to this embodiment, each silicon steel sheet is provided
with a plurality of magnetic poles radially extending outwardly
from the outer periphery of the sheet and these magnetic poles are
alternately provided with slots to accommodate the respective
permanent magnets so that the sides of these permanent magnets
facing the rotary shaft may have the same polarity. With such
arrangement, mutual repulsion of the diametrically opposed magnetic
poles provides the rotor with magnetic poles twice the number of
permanent magnets.
In addition, the permanent magnets are inserted into the respective
slots and radially held in the highly permeable material so that
accidental flying-off of the permanent magnets due to high speed
rotations is avoided. Accordingly, the need for the member for
covering the outer periphery of the rotor so as to prevent the
permanent magnets from flying off is eliminated, and the iron loss
due to the use of such a member can be eliminated. The iron loss
can be further reduced by making the yoke from the laminated steel
sheets.
Moreover, the permanent magnet according to this embodiment has a
simple configuration and requires no high precision in finishing
the surface, facilitating the manufacture of the permanent
magnet.
The second embodiment of the invention will be described in
reference with FIG. 5.
In FIG. 5, the lines of magnetic force exiting from the N-pole of
the permanent magnet 30 partially return to the S-pole without
passing through the pole-face of the magnetic pole 24b due to a
so-called magnetic flux leakage and, therefore, the total amount of
magnetic flux on the pole-face of the magnetic pole 24a will exceed
that of the magnetic pole 24b if the pole-face of the magnetic pole
24a has a width W1 equal to a width W2 of the magnetic pole 24b. In
view of this, the width W2 is made larger than W1 in the instant
embodiment so that the total amounts of magnetic flux on the pole
faces of the magnetic poles 24a and 24b are equalized thereby
making the generated torque uniform.
The third embodiment will be described with reference to FIGS. 6
and 7.
In FIG. 6, the magnetic poles 24a and 24b are provided with slits
33 oriented in conformity with directions of magnetism presented by
the respective magnetic poles 24a and 24b. Generally, the lines of
magnetic force exiting from the N-pole reach the S-pole along the
shortest paths and accordingly the magnetic flux density at
opposite ends is higher than that of the middle portion on the
pole-face of the magnetic pole 24a, as compared with that of the
embodiment of FIG. 4. To overcome such undesirable tendency, the
magnetic pole in the instant embodiment is provided with the slits
33 so that the lines of magnetic force are forcibly guided to exit
from or enter into the pole-face along the slits 33.
FIG. 7 shows a pattern of the lines of magnetic force in this
embodiment. As shown, the lines of magnetic force exiting from the
N-pole of the permanent magnet 30 are guided by the slits 33 of the
magnetic pole 24b prior to passing through the driving coils 6,
then guided by the slits 33 of the magnetic pole 24a and return to
the S-pole of the permanent magnet 30. Thus, the lines of magnetic
force are distributed uniformly by the slits 33 on the same
pole-face, thus making the generated torque uniform. In this way,
not only the thermal distribution in the permanent magnet rotor is
improved but also the cooling area is increased.
The fourth embodiment of the invention will be described with
reference to FIG. 8.
FIG. 8 shows, in section, a 6-pole rotor 7. The respective magnetic
poles 24a, 24b, and 24c radially extending outwardly and angularly
spaced from each other by 60.degree.. These magnetic poles are
alternately provided with permanent magnets 34a, 34b, and 34c
inserted thereinto so that the N-poles of these magnets face
inwardly. The yoke 21 is centrally provided with the opening 27 to
accommodate the rotary shaft and the opening 27 is provided with
the keyway 28 to prevent the rotary shaft from being rotated with
respect to the rotor.
The respective permanent magnets 34a, 34b, and 34c are arranged
with their N-poles facing inwardly in this embodiment and,
therefore, the lines of magnetic force exiting from the respective
N-poles are repelled by the N-poles of the other permanent magnets
and enter through the adjacent pole-faces into the respective
S-poles. In this manner, the magnetic poles containing the
associated permanent magnets are S-polarized while the magnetic
poles containing no permanent magnet are N-polarized.
While this embodiment employs the permanent magnets made of cast
praseodymium (Pr) alloy, it is possible to employ the permanent
magnets made of any type selected from the group consisting of cast
type (e.g. alnico or praseodymium magnet), sintered type (e.g.
ferrite or rare earth magnet), and resin bound type (e.g. ferrite
or rear earth magnet).
The permanent magnets are made in the form of a rolled plate having
a rectangular cross-section and a length in the axial direction two
to five times the width in the circumferential direction of the
rotor. The rectangular cross-section facilitates manufacturing
process of the permanent magnets in comparison with the
conventional arcuate ones. In addition, since the permanent magnets
according to the invention are not bonded to the outer peripheral
surface of the yoke, there is no need for precision surface
finishing, Moreover, the permanent magnets of the invention never
fly off under the influence of high speed rotations because they
are fitted in the associated slots 25 and held radially between the
highly permeable materials. Thus, the rotor according to the
invention is useful for the high speed electromotor.
The silicon steel sheets 22 of the yoke 21 are formed by a press
and thereby not only a high productivity is achieved but also the
rotor of an accurate outer dimension can be obtained, thus allowing
an efficient electromotor to be realized.
The fifth embodiment of the invention will be described, in which
the bridges 26 are provided with grooves to limit passage of the
magnetic flux.
In FIG. 9, which shows part of the magnetic pole 24a in an enlarged
scale, a part of the magnetic flux exiting from the N-pole of the
permanent magnet 30 passes through the bridge 26 to the S-pole of
the permanent magnet 30 as shown in the figure. This part of
magnetic flux passing through the bridge 26 never passes through
the space external to the yoke 21 and, therefore, never intersects
the stator of the electromotor. Consequently, no force for
rotational driving of the rotor is generated. By minimizing the
amount of magnetic flux passing through the bridge 26, the magnetic
force of the permanent magnet 30 can be utilized with higher
efficiency.
The part of magnetic flux .phi. passing through the bridge 26 can
be calculated according to the following equation:
where S is the sectional area of the bridge 26 and B is the
magnetic flux density in the silicon steel sheet 22.
It is obvious from this equation that the sectional area S of the
bridge 26 may be reduced to minimize the amount of magnetic flux
passing through the bridge 26.
In this embodiment, each bridge 26 is provided with a flux limiting
groove 26a. Formation of such groove 26a correspondingly reduces
the sectional area of the bridge 26 and thereby limits the amount
of magnetic flux passing through this bridge 26.
Provision of the above-mentioned flux limiting grooves 26a in the
respective bridges 26 allows the amount of magnetic flux passing
through the bridges 26 to be effectively limited so that the
magnetic force of the permanent magnets can be efficiently utilized
and thus the permanent magnet rotor of a higher efficiency can be
obtained.
Formation of the flux limiting grooves 26a is performed by
successive steps of stamping out the individual silicon steel
sheets 22, laminating them to form the yoke 21 and finally
providing the yoke 21 by use of suitable tools such as a grinder
with the desired grooves 26a. The manufacturing process of the
grooves 26a is easier than the stamping of the silicon steel sheets
22 so far as the dimensional accuracy is concerned, and therefore,
it is possible to provide the respective bridges 26 with minimized
sectional area, respectively, without difficulty. Accordingly, the
permanent magnet rotor of the invention is advantageous in
comparison with the conventional permanent magnet rotor having no
grooves formed in the respective bridges because of the facilitated
production and the bridges having the significantly reduced
sectional areas both achieved by the present invention.
The sixth embodiment of the invention will be described, in which
each of the bridges 26 is provided only on the front side of the
associated slot 25 as viewed in the direction of rotation.
In FIG. 10, the respective slots 25 are of the respective slots 25
are of half-closed type. More specifically, the bridges 26
connecting the respective magnetic pole pieces 26b to roots of the
respective magnetic poles are of the cantilever type and configured
point-symmetrically with respect to the rotational axis so as to
have the respective bridges 26 on the front sides but no bridges on
the rear sides in the rotational direction.
The permanent magnets 30 and 31 are inserted in the axial direction
into the respective slots 25 of the yoke 21 made from the laminated
silicon steel sheets. The silicon steel sheets are provided on the
sides having no bridges with stoppers 26c to prevent the respective
permanent magnets 30 and 31 from flying off from the slots during
high speed rotations.
FIG. 11 shows a pattern of magnetic flux produced under a load
torque on the basis of field analysis. A width of the bridge 26
corresponds to a width of leakage flux and a thickness of the
permanent magnet 30 corresponds to a thickness of the leakage flux
occurring at opposite ends.
In this manner, as shown in FIG. 11, the leakage flux flows through
the bridge 26 and the flux is saturated in the bridge as well as in
the porion of the magnetic pole piece extending adjacent to the
bridge. Consequently, even under the load current, the magnetic
flux exiting from the permanent magnets 30 and 31 is not readily
deflected by the magnetic pole piece 26b and, accordingly, the
circumferential center of the magnetic pole is not readily moved
under the load. Such condition is convenient for introduction of
so-called sensorless technique. The left half portion of each
magnetic pole piece with respect to the center thereof generates a
larger amount of magnetic flux, since no leakage flux occurs on the
side having no bridge, and, as a result, the leakage flux occurring
along the bridge will not significantly reduce the total amount of
magnetic flux.
The seventh embodiment of the invention will be described. In this
embodiment, laterally opposite ends 30a and 31a of the respective
permanent magnets 30 and 31 facing the bridge 26 as well as axially
opposite ends 30b and 31b of these permanent magnets are covered
with a nonmagnetic material.
In FIG. 12, there are provided spacers 32 made of aluminum or
nonmagnetic stainless in the form of a frame to cover the laterally
opposite ends and the axially opposite ends of the respective
permanent magnets 30 and 31. Each frame is dimensioned so as to
accommodate the associated magnet snugly and has a height slightly
less than a thickness of the magnet so that the magnet is not
protruded over the frame too much.
Assembling is performed by inserting the permanent magnets 30 and
31 into the associated spacers 32 in the magnetizing direction
followed by inserting the respective subassemblies into the slots
25 of the yoke 21, to obtain a configuration shown in the sectional
view of FIG. 13.
FIG. 14 shows a flow of magnetic flux observed after the assembly
is incorporated into the magnetic circuit. The nonmagnetic spacers
32 provided on the laterally opposite ends of the permanent magnet
30 serves to obstruct a flow of magnetic flux and thereby to assure
that the magnetic flux exits from the magnet substantially without
leakage. In this manner, the leakage flux can be reduced and the
effective gap flux can be maintained.
Additionally, the height of each spacer is dimensioned to be
slightly larger than the thickness of the permanent magnet
accommodated therein and this dimensioning is advantageous in that,
when the magnet is inserted together with the associated spacer
into the slot of the yoke, the magnet is never in contact with the
slot of the yoke. As a result, a fear that the magnet surface might
be damaged and rusted can be eliminated.
the eighth embodiment of the invention will be described.
In FIG. 16, this embodiment is characterized by an arrangement such
that the yoke 21 has an opening 15 axially extending therethrough
to receive the rotary shaft 8. The opening 15 has a diameter larger
than the outer diameter of the rotary shaft 8. The rotary shaft 8
is received by the opening 15 substantially in concentric
relationship with a flux-leakage-proof member 16 interposed between
the outer peripheral surface of the rotary shaft 8 and the inner
wall of the opening 15 so that the yoke 21 is integrally bonded to
the rotary shaft 8 with interposition of this flux-leakage-proof
member 16.
More specifically, as shown in FIG. 15, the magnetic flux developed
in the rotor 7 of the brushless electromotor as recited in claim 1
passes through the space external to the rotor 7 under the
inter-pole repulsion effect of the permanent magnets 30 and 31, and
then intersects a stator core 17. Magnetic poles of this stator
core 17 generates a magnetic field to be rotated under the effect
of current flowing through the driving coils 6. The permanent
magnet rotor 7 is rotationally driven by the rotating field
generated by the magnetic poles of the stator core 17.
The permanent magnet rotor 7 has the opening centrally extending
through the yoke 21 and having the inner diameter substantially
corresponding to the outer diameter of the rotatable shaft 8 to
receive the rotary shaft 8. to assemble the permanent magnet rotor
7, the yoke 21 is heated so as to thermally expand the opening 15
and the rotary shaft 8 is forced thereinto. Then, the yoke 21 is
cooled to bring the inner wall of the opening to close contact with
the outer peripheral surface of the rotary shaft 8 and thereby to
bond the rotary shaft 8 integrally to the yoke 21.
In assembling the permanent magnet rotor having balance weights on
axially opposite end surfaces of the yoke 21, these balance weights
are manufactured in a separate process and thereafter the rotatable
shaft is forced into the opening extending through the balance
weights and the yoke.
With the previously mentioned embodiment of permanent magnet for
brushless electromotor, there is a fear that the magnetic flux
partially passes through the inside of the rotary shaft and then
leak externally of the axially opposite end surfaces of the
permanent magnet rotor. Partial leakage of the magnetic flux will
prevent the magnetic flux from intersecting the stator core and, in
consequence, the magnetic force of the permanent magnets do not
effectively contribute to rotation of the electromotor, resulting
in the reduced efficiency of the brushless electromotor.
Accordingly, it is an object of this eighth embodiment to provide a
permanent magnet rotor for brushless electromotor so improved that
the flux leakage otherwise often occurring outwardly from the
axially opposite end surfaces of the rotor is substantially avoided
and manufacturing the permanent magnet rotor is facilitated.
As shown in FIG. 16, the permanent magnet rotor 7 includes the
central opening 15 axially extending therethrough to receive the
rotary shaft 8, the flux-leakage-proof member 16 made of aluminum
die cast material between the yoke 21 and the rotary shaft 8, and
the balance weights 18 made of aluminum die cast material on the
axially opposite end surfaces of the yoke 21. The aluminum die cast
material has a flux shielding property by which the magnetic flux
is prevented from passing through both the member 16 and the
balance weights 18. Consequently, the magnetic flux passing through
the rotary shaft 8 toward the axially opposite ends is shielded by
the flux-leakage-proof member 16 and the balance weights 18 and
cannot exit outwardly from the axially opposite end surfaces of the
yoke 21.
When such permanent magnet rotor 7 is used for the brushless
electromotor, all the magnetic flux lines pass through on the faces
perpendicular to the rotary shaft 8 and effectively intersect the
stator core 17. Upon current supply to the driving coils 6, a
rotating field is generated in the magnetic poles of the stator
core 17 so that the permanent magnet rotor is rotationally driven
by interaction between the rotating field generated in the stator
core and the magnetic flux in the permanent magnet rotor.
Accordingly, the greater the amount of magnetic flux intersecting
the stator core is, the larger the torque is. With this specific
embodiment of permanent magnet rotor 7, the magnetic flux thereof
can be effectively converted to the rotating force, since all the
magnetic flux generated by the permanent magnets 30 and 31
intersects the stator core 17 without exiting externally from the
axially opposite end surfaces of the yoke 21.
Furthermore, this embodiment of permanent magnet rotor 7 includes
the rotary shaft 8 loosely inserted into the opening 15 axially
extending therethrough, the flux-leakage-proof member 16 and the
balance weights 18, wherein the components 16 and 18 are integrally
molded from aluminum die cast material. Such unique arrangement
advantageously makes it possible to eliminate not only a separate
process of manufacturing the balance weights but also a process of
assembling these balance weights together with the yoke into the
permanent magnet rotor, and thereby facilitates manufacturing of
the permanent magnet rotor 7.
Although this embodiment has been described as including the
balance weights, even when the permanent magnet rotor is provided
only with the flux-leakage-proof member, the magnetic flux is
effectively prevented from passing through the rotating shaft so as
to improve the efficiency of the electromotor.
Moreover, the flux-leakage-proof member is not limited to that made
of aluminum die cast material, and a similar effect can be achieved
even when the member is made of the other material having a lower
magnetic permeability, such as resin.
The ninth embodiment will be described. This embodiment is
characterized by a unique arrangement that the inner peripheral
surface of the yoke 21 defining the slot 25 is provided with
projections for engagement with the associated permanent magnet 30
and 31 forced into this slot.
In FIGS. 18 and 19, the inner peripheries of the silicon steel
sheets 22 defining the slots 25 are provided with a plurality of
edges 36 each defined by two sides of a triangle projecting
inwardly of the slot 25.
The permanent magnets 30 and 31 come into engagement with tips of
the respective edges 36 as they are forced into the respective
slots 25, and are thereby held within the respective slots 25. By
means of the edges 36, the permanent magnets are not in surface
contact with the inner peripheral surface of the respective slots
25. Accordingly, the permanent magnets 30 and 31 can be forced into
the respective slots 25 with a small force without significant
frictional resistance due to the contact between the permanent
magnets 30 and 31, and the slots 25.
After forced into the respective slots, the outer surfaces of the
permanent magnets are firmly engaged with the tips of the
respective edges as shown in the drawings, and the permanent
magnets are reliably prevented from dripping off. In this
embodiment of permanent magnet rotor 7, no adhesive is used to hold
the permanent magnets 30 an 31 within the respective slots 25 and,
therefore, there is no fear that the permanent magnets drop off due
to dissolution of adhesive in refrigerant or pressurized fluid when
the electromotor is used in such refrigerant or pressurized
fluid.
In FIG. 20, the pressed recesses 23 used to laminate the silicon
steel sheets 22 together are provided adjacent to the inner
peripheries of the silicon steel sheets 22 defining the respective
slots 25. The pressed recesses 23 are formed by partially pressing
the silicon steel sheets by means of a metal molding press.
Formation of these pressed recesses adjacent to the inner
peripheries allows this peripheral edge to be deformed by a
pressure of the metal mold press so as to form edges 36 projecting
inwardly from the respective slots 25. In this way, the process
required from formation of the projecting edges 36 is partially
eliminated, facilitating manufacturing the permanent magnet
rotor.
FIG. 21 partially shows the yoke in a variant of this embodiment.
In this variant, each of the edges 36 formed in the silicon steel
sheets 22 has a triangular projection 37 for engagement with the
permanent magnet (not shown) and notches 38 formed on both ends of
the base of the triangular projection 37. The base of the
triangular projection 37 is located in the inner periphery of the
silicon steel sheets 22 defining the slot 25 and in the yoke 21.
The triangular projection 37 is connected to the inner periphery of
the silicon steel sheets defining the slot by the notches 38.
To firmly engage the permanent magnet, the triangular projection of
each edge must have a vertical angle smaller than a predetermined
degrees and a predetermined height. Excessively large vertical
angle of the triangular projection will require a correspondingly
large force to insert the permanent magnet into the slot. If the
edge does not have the predetermined height, the edge will be
deformed as the permanent magnet is forced into the slot and will
not function as expected. However, providing the inner periphery of
the silicon steel sheets defining the slot with the edge having the
desired vertical angle and height reduce the cross sectional area
of the permanent magnet which can be forced into the associated
slot or enlarged the slot. This is contradictory to the demand for
compactness and high efficiency of the brushless electromotor.
The edge 36 in this specific embodiment includes, as mentioned
above, the triangular projection 37 and the pair of notches 38.
Such arrangement not only facilitates insertion of the permanent
magnet without enlarging the slot 25 or reducing the sectional area
of the permanent magnet but also prevents the permanent magnet from
accidentally dropping off by effective engagement with the
permanent magnet after the latter is forced into the slot.
Though the present embodiment has been explained as including the
triangular projection adapted to be engaged with the permanent
magnet, the shape of the projection is not limited to the triangle
and, for example, the projection having a small radius
semi-circular tip may also be used.
Furthermore, the yoke is not limited to that consisting of the
laminated silicon steel sheets and, for example, the yoke may also
be of integral metallic block having the slot into which the
permanent magnet is forced and the projections formed on the inner
wall of the slot adapted to be engaged with the permanent
magnet.
The tenth embodiment of the invention will be described. This
specific embodiment is characterized by an arrangement such that
after the permanent magnets 30 and 31, which are shorter than the
axial length of the yoke 21, are inserted into the respective slots
25, resultant cavities defined within the respective slots are
filled with suitable putty having weighs selected depending on a
gravity center displacement of the object to be rotationally driven
and the balance weights 39 are formed as the putty is cured or
hardened.
FIG. 22 shows the permanent magnet rotor 7 provided with the
balance weights formed in the above-mentioned manner and, in this
permanent magnet rotor 7, the respective permanent magnets 30 and
31 are shorter than the axial length of the yoke 21 so as to define
the cavities within the respective slots 25. As shown in the
figure, these cavities are filled with the putty composed of fine
metallic powder mixed with resin and this putty is hardened or
solidified to form the respective balance weights 39.
The yoke 21 of permanent magnet rotor 7 contains these balance
weights 39 on the sides of the respective permanent magnets 30 and
31 which are axially opposed to each other and, therefore, the
gravity center of the yoke 21 is displaced toward the axially
opposite ends of the yoke 21. Consequently, a vibration mode tuning
with would other wise occur in a total system including the rotary
shaft and the eccentric rotor can be avoided and vibrations of the
eccentric rotor due to its rotation can be absorbed.
In FIG. 22, the balance weights 39 are weight-adjusted so as to
fulfill such purpose. More specifically, a ratio of metallic powder
to resin is so selected that the balance weights 39 may compensate
the vibration mode tuning possibly occurring in the total system
comprising the rotary shaft and the eccentric rotor, with the
respective cavities in the slots 25 being filled with the putty
mixed at this ratio. Alternatively, the amount of putty is adjusted
for each balance weight 39 and the respective cavities are filled
with different amount of putty which defines the balance weight 39.
It should be understood that the putty is not limited to the
mixture of metallic powder and resin, but aluminum die cast
material may also be used.
As described above, the balance weights for this embodiment of the
permanent magnet rotor are formed within the yoke and have no
portions projecting outwardly from the axially opposite ends of the
yoke. Therefore, the balance weights encounter no fluid resistance
during rotation. Additionally, there is no fear that the balance
weights fly off under a centrifugal force due to rotation of the
permanent magnets since the balance weights are provided within the
yoke. This makes it possible to obtain the permanent magnet rotor
which is excellent in the rotational drive efficiency and free from
the fear that the balance weights might accidentally fly off during
operation.
Finally, the eleventh embodiment of the invention will be
described. This embodiment is characterized in that the rotor
contains therein a cooling mechanism.
In FIG. 23, a pair of heat pipes 19 are embedded in the yoke 21 to
be adjacent to the respective permanent magnets 30 and 31, and
these heat pipes 19 are filled with operating fluid by which a heat
exchange occurs. More specifically, a heat absorbing section of
each heat pipe 19 lying within the yoke 21 absorbs internal heat of
the yoke and a radiating section projecting outwardly from the yoke
radiates the internal heat. The operating fluid cooled by heat
exchange with atmosphere returns to the heat absorbing section of
the heat pipe 19. In this manner, the heat pipes 19 continuously
radiate the internal heat of the permanent magnets as well as the
yoke to the outside, cooling the permanent magnet rotor.
FIGS. 24 and 25, there is shown a variant of this embodiment, in
which the rotary shaft 8 serves also as the heat pipe. Such
arrangement provides the permanent magnet rotor which is fully
closed and able to radiate the internal heat to the outside. It
should be understood that such arrangement can utilize sintered
alloy, massive iron or cold rolled steel (SPCC) for the yoke.
Now a method of making the rotor according to the invention will be
described in detail.
As apparent from the above description, the permanent magnet rotor
of the invention is manufactured by the method generally includes
the steps of forming the yoke provided with the slots adapted to
receive the respective permanent magnets, forming the permanent
magnets each configured to match the respective slots of the yoke,
and forcing these permanent magnets into the respective slots of
the yoke.
More specifically, the yoke 21 and the permanent magnets 30 and 31
are separately manufactured, then these permanent magnets thus
manufactured are inserted into the yoke to form the permanent
magnet rotor 7. The yoke 21 is formed by laminating a plurality of
silicon steel sheets 22 together and each silicon steel sheet 22 is
configured by stamping out to have the magnetic poles 24 (24a, 24b,
24c) along the outer periphery and the openings inside the magnetic
poles through which the respective permanent magnets extend. In
addition, each silicon steel sheet 22 is provided with the
rectangular pressed recesses 23 formed by pressing with the metal
die.
The pressed recesses 23 of the respective silicon steel sheets 22
may be forced one into another to laminate the silicon steel sheets
integrally and thereby to form the yoke 21. The openings of the
respective silicon steel sheets 22 are aligned with one another to
from together the respective slots 25 to receive the associated
permanent magnets 30 and 31.
The permanent magnet are manufactured by a method generally
includes the steps of mixing powder of suitable magnetic material
with epoxy binder, pouring this mixture into a mold and molding the
mixture in a magnetic field into a given configuration; curing this
molded mixture by heat treatment; and surface-cutting this molded
and cured mixture to be conformed to the slot 25 of the yoke
21.
The permanent magnets obtained in the above mentioned manner are
forced into the respective slots 25 of the yoke 21 to complete the
permanent magnet rotor 7.
Another method of making the permanent magnets for the brushless
electromotor will be described, which is devised particularly to
facilitate manufacturing of the permanent magnet rotor containing
therein the permanent magnets of a relatively complicated shape
without damaging the permanent magnets during assembling.
This alternative method includes the steps of providing the slots
extending through the yoke, filling the slots with powder of
suitable magnetic material mixed with epoxy binder,
compression-molding this mixture in a magnetic field applied in the
radial direction in relation to the rotary shaft of the rotor and
curing the compression-molded mixture with heat treatment to form
the permanent magnets directly within the slots of the yoke.
FIG. 26 shows the process of forming the permanent magnets within
the slots. As shown, the yoke 21 is placed on a pedestal 42 having
coils 41. On the top of the yoke 21, there are placed jigs 40
provided with openings 43 each having the same configuration as
that of the slot 25 and compressing pistons 44. Each jig 40 is
additionally provided with a coil 45. The slots 25 of the yoke 21
is filled with a raw material 46 of the permanent magnets composed
of the magnetic powder mixed with the epoxy binder. In view of the
fact that the raw material 46 of the permanent magnets has its
volume reduced as it is compressed, each slot is filled with an
amount of the raw material larger than the volume of the slot so
that the raw material 46 is partially pressed into the opening 43
of the jig 40.
Electric current is applied to the coils 41 and 45 to generate
magnetic flux extending, as shown, through the center of the yoke
21, then through the respective slots from the inside toward the
outside and to thereby provide a magnetic field intersecting the
raw material 46 of the permanent magnets filling the respective
slots 25. The compressing pistons 44 are then forcibly moved as by,
for example, oil pressure in a direction indicated by P in FIG. 26
and thereby the raw material 46 is compression-molded into the
permanent magnets.
After the compression-molding, the yoke 21 is removed from the jigs
40 and the pedestal 42, and subjected to the heat treatment at a
temperature of 100.degree. C. to 150.degree. C. to cure or harden
the permanent magnets contained therein.
In this way, the permanent magnets are formed within the respective
slots 25 of the rotor 7, as shown in FIG. 4 and other Figs. These
permanent magnets have their inner sides magnetized as S-poles and
their outer sides as N-poles under the effect of the magnetic field
applied during the compression-molding. The permanent magnets 30
and 31 are placed with their magnetic pole surfaces having the same
polarity being opposed to each other so that their mutual repulsion
causes the magnetic flux to extend from the magnetic pole 24a to
the magnetic pole 24b of the yoke 21, as illustrated. This magnetic
flux intersects the stator (not shown) of the electromotor provided
in the proximity of the outer periphery of the yoke and interaction
between the magnetic flux and the stator causes the permanent
magnet rotor 7 to be rotationally driven.
As mentioned above, this embodiment of the method for making the
permanent magnet rotor allows the permanent magnets to be easily
manufactured without damaging the magnets as they are forced into
the respective slots of the yoke, because the permanent magnets are
formed directly within the yoke. Furthermore, this unique method of
the invention allows the permanent magnets of a relatively
complicated shape to be formed within the yoke and such feature
presents a significant effect particularly for so-called skewed
permanent magnet rotor in which the yoke has the magnetic poles
gradually displaced along the axial direction of the permanent
magnet rotor.
In FIG. 27, the skewed permanent magnet rotor 7 is obtained by
laminating a plurality of silicon steel sheets 22 with the
individual silicon sheets being successively displaced by a small
angle around the rotary shaft 8. The respective slots 25 defined in
such permanent magnet rotor 7 provided helical curves. Even within
the slots 25 of such complicated shape, the permanent magnets 30
and 31 can be directly formed by the same method as described above
in reference with FIG. 26.
Although the method of the invention described above particularly
for the permanent magnet rotor in which the yoke contains therein a
pair of permanent magnets and is provided along the outer periphery
with hour magnetic poles alternately magnetized in N- and
S-polarities by mutual repulsion of the pair of permanent magnets,
the method of the invention is not limited to production of such
permanent magnet rotor having the constitution as mentioned above.
Namely, the method may be utilized to form the permanent magnets
directly within the yoke also in the permanent magnet rotor having
along the outer periphery an optional number of magnetic poles or
even in the permanent magnet rotor having the permanent magnets
provided in association with the respective magnetic poles.
Although the method of the invention has been described above on
the assumption that each permanent magnet has the rectangular
cross-section, the permanent magnet may have any cross-sectional
shape.
The method described above allows the permanent magnet of high
efficiency to be easily obtained without need for surface-cutting
of the permanent magnets during the manufacturing process. In
addition, the method of the invention allows the permanent magnets
to be efficiently used because there is no fear that the permanent
magnets might be damaged during the manufacturing process.
The method of the invention can be employed for the slots of
relatively complicated configurations, since the permanent magnets
are formed directly within the respective slots of the yoke. The
method of the invention not only allows the permanent magnets of
relatively complicated cross-sectional shapes to be easily formed
but also allows even the permanent magnet rotor having the curved
slots to be easily manufactured.
Industrial Usefulness
As apparent from the above description, the permanent rotor
according to the invention is useful for the brushless electromotor
having a simplified structure and high efficiency and suitable for
high speed operation.
* * * * *